There are many types of touch sensitive computer input devices currently used for the purpose of digitizing touch on or in conjunction with computer displays. Such devices measure the position of a stylus or finger touch on the sensor surface. The position is used to generate coordinates for the purpose of interacting with the computer, for example in pointing to icons on the display, picking menu items, editing computer generated images, and feedback for input of hand-drawn characters and graphics.
Such devices which sense a human touch may sense using any number of technologies, including capacitive sensing, resistive sensing using a conductive overlay sheet, infrared sensing, acoustic wave sensing, and piezoelectric force sensing. Digitizers which use corded hand held styli such as pens or pucks typically use electromagnetic sensing, electrostatic sensing, resistive sensing, or sonic pulse sensing.
Devices responsive to human touch are typically used for cursor control application, for example pointing to display icons and picking menu items. Devices that are responsive to styli (usually a corded pen) are used to create or trace drawings, blueprints, or original art. These devices are also used for character or handwriting recognition. It is desirable that the device have a pen and paper feel so that it's use is intuitive to most users. It is therefore desirable that the sensor reproduce the trace of the pen below the stylus by some visual means so that the user has visual feedback.
Some of these devices are responsive to both human touch and stylus touch, thereby providing the convenience of stylus-based input, for example when writing on the screen, as well as the ease of human touch input, which does not require the user to find the stylus and pick it up to use it. However, because these sensors cannot distinguish between human and stylus touch, the user may not touch the screen while using the stylus, or vice versa. Accordingly, in the use of these sensors the operator must take great care, which detracts from their desirability.
Some such devices are used as computer input tablets which, rather than being placed on the face of a display, are placed on the desk top next to the computer, similar to a mouse. Such sensor devices are commonly employed for handwriting recognition, in which they are used as a writing tablet. However, when using such tablets, the operator must painstakingly avoid touching the screen with his finger or hand while writing with the stylus. Accordingly, these devices are rather awkward to use.
One system that has the capability of sensing both stylus and human touch is disclosed in U.S. Pat. No. 4,686,332 (Greanias et al., Aug. 11, 1987). The device uses an X-Y array of discrete conductors in spaced planes to electrostatically detect the position of the stylus; finger touch position is determined by detection of a change in capacitance of the conductors closest to the finger. The two large arrays of closely spaced conductors required for good resolution, however, is difficult to fabricate, requiring etching of two layers of conductive material into parallel conductor patterns, and then careful placement of the layers one over the other to accomplish the spaced X-Y conductor array. Having two layers of conductors over the display also interferes significantly with light transmittance, making the device uncomfortable to use. In addition, each conductor or pair of conductors requires discrete electronic components to make the capacitance measurements, making the device complex and costly. Finally, the requirement of driving the conductors individually results in relatively slow digitization response, unless expensive high-speed drive and sensing circuitry is used.